After his graduation in 1934, he served a short time in the Dutch East Indies before he entered service in the Royal Netherlands Navy Submarine Service. On 13 May 1940, De Jong sailed to England on board the Dutch HNLMS O 24 and during World War II he saw action initially as first officer and from mid-1944 onwards as Commanding officer of that vessel. He lived in England during the World War II. In April 1946 he returned on board the HNLMS O 24 to the Netherlands. In 1947, he was attached to the Naval Staff of the Admiralty In 1948, he became adjutant to the Navy Minister.

During that time the independence of the Royal Netherlands Navy in the Armed forces of the Netherlands was in question, the main focus of the discussion was the new command structure for all the service branches. Both a horizontal-structure and vertical-structure had their proponents and opponents, the Royal Netherlands Navy was historically a proponent of a vertical-structure but the horizontal-structure received increasing support, including from the then Minister of War and the NavyKees Staf. During the cabinet formation for the Cabinet de Quay the decision was made to go with the horizontal-structure and the first step was made with the combining of the Ministry of War and the Ministry of the Navy. With the combining of the ministries a serious efficiency problem in the Ministry of War was discovered, while the Ministry of the Navy was thought to be in an excellent administrative condition.

After his meeting with Admiral Brouwer, De Jong reported to the incoming Minister of DefenceSidney J. van den Bergh, who formally asked De Jong to become the new State Secretary for Defence. De Jong was given a few days to think it over and he used that time to speak with Carl Romme. De Jong informed him that he was not a member of the Catholic People's Party but Romme respond by saying: "You are not a member of a different party? No, but you are a Roman Catholic? Then we will we see you as one of us." Romme did advise De Jong to become a member of the Catholic People's Party after six months. On 11 June 1959, De Jong reported back to Van den Bergh and was told about the new horizontal-structure, De Jong who was a proponent of a vertical-structure was left with a few remarks. The next day the meeting was continued in the presence of the incoming Secretary-General for the Ministry of Defence Duyverman who was going to be responsible for all financial and budgetary affairs for the Ministry of Defence. De Jong objected, he thought he could not be politically responsible as the State Secretary for Defence if he wasn't responsible for all financial affairs for his own department, Van den Bergh became irritated with De Jongs position on the matter and the meeting was adjourned for 16 June 1959.[1]

His cabinet was confronted with a demand for democratic reforms in the society and it decided to democratise colleges and universities after the famous Maagdenhuisbezetting. Plans were made to modernise politics by establishing an electoral system with districts or a chosen Prime Minister, but these plans were not implemented. Meanwhile, a pay pause due to the decision of employers and employees to raise wages was partly revoked after anti-government demonstrations and strikes. The Minister of Economic AffairsLeo de Block resigned, officially as a protest against the wage rise in the metal industry, but another reason was his slow reaction to the inflation and rising prices after the introduction of Value added tax. More unrest took shape in demonstrations against the Vietnam War. Internationally, relations with Indonesia improved, resulting in a visit by president Suharto which was, however, overshadowed by the occupation of the Indonesian embassy by Moluccans. The Soviet Union invasion in Czechoslovakia was seen as a reason to increase the defence budget. During his term as Prime Minister of the Netherlands De Jong met with numeral state leaders, including then PresidentRichard Nixon, Presidents of FranceCharles de Gaulle and Georges Pompidou. He was present in the hospital when Prince Claus announced the birth of his son Willem-Alexander, Prince of Orange on 27 April 1967.

Following the end of his active political career, De Jong occupied many posts in industry and seats on numerous supervisory boards. In December 1990 he was selected to lead a diplomatic mission to Iraq to free Dutch hostages, but ultimately the mission was cancelled.

De Jong was present at the ascension of Willem-Alexander to the Dutch throne on 30 April 2013. He was Prime Minister at the time of the King's birth. At the age of 98 he was the oldest person attending the ceremony.[20]

On 1 December 2013, De Jong and fellow former prime ministers Dries van Agt and Ruud Lubbers were present at the presentation of the book Polarisatie en hoogconjunctuur, a book about Dutch politics in the 1960s. De Jong received the first copy from incumbent Prime Minister Mark Rutte.[21][22]

During his time as Prime Minister of the Netherlands, he was accused of not showing enough strength and too little action when needed during the social revolution of the 1960s by some of the counterculture of that time. De Jong was frequently labelled as old-fashioned, and that old-fashioned image was reinforced by his bowler hat, which he wore frequently outside, a custom he picked up during his time living in England during World War II. In contrast the Silent majority were reported to be content with the policies of his cabinet. More than forty years later, De Jong is considered by many to have made well thought-out decisions during his time as Prime Minister and when needed showed enough strength with a far more progressive ideology than he was credited for at the time.

Politicians from the entire political spectrum have labelled De Jong as one of the better prime ministers of the Netherlands. In an episode of the Dutch television programme Netwerk aired in 2005, he is even labelled as perhaps the best postwar Prime Minister of the Netherlands. This view has been shared by numerous high-profile current and former Dutch politicians, including former Prime Minister Dries van Agt[23] and former Deputy Prime MinisterHans Wiegel, who praised De Jong being perhaps the best Prime Minister after World War II.[24] Former Member of the Senate and Righteous Among the NationsJohan van Hulst said that of all the prime ministers he worked with in twenty-five years, De Jong was the best.[25] On 23 March 2011, the history channel Hollanddoc devoted an episode about his life with the quote "Attention for the least-known prime minister after World War II, but perhaps the most successful: Piet de Jong"[26] In November 2011, the second edition of his in 2001 released biography Van buitengaats naar Binnenhof. P.J.S. de Jong (From offshore to Binnenhof. P.J.S. de Jong) became well received and further enhanced his reputation: "He showed himself a team leader with perspective and humor. The ideal manager in a time of crisis of authority, polarization and social unrest."[27]

De Jong said to Queen Juliana, for whom he had served as aide-de-camp, when he was sworn in as State Secretary for Defence, "Majesty, there you see how a person comes down in the world." (Dutch: "Majesteit, zo ziet u maar hoe een mens aan lager wal kan raken.") And in a response to a delicate question of a radio reporter, when asked what his thoughts on pornography were, he said, "As far as I know, pornography is the only working medicine against seasickness." When later a Belgian minister spoke negatively about the libertarian ideas of the Dutch on pornography, he responded, "Well, the Belgians aren't a seafaring people, are they?"[28]

After World War II, De Jong married former resistance fighter Anna Geertruida Jacoba Henriëtte "Anneke" Bartels (8 January 1915 – 6 January 2010). At the time when they met Bartels was serving in the women's division [nl] of the Royal Netherlands Navy. They married on 26 June 1947 and had one daughter and two sons, Maria (born 31 May 1948) Jos (born 31 August 1949) and Gijs (born 15 October 1952). Anneke Bartels died on 6 January 2010 two days before her ninety-fifth birthday after suffering from heart and lung illnesses since 2000.[29][1][30]

Piet de Jong died on 27 July 2016 at his home in The Hague at the age of 101. Having survived his wife by six years, De Jong was survived by his three children, eight grandchildren and fifteen great-grandchildren.[32]